Solar cells convert light energy into electric energy. A solar cell has a PN junction between the P-type semiconductor and the N-type semiconductor. While illuminated by sunlight, the PN junction generates a great amount of free electrons and electron holes. The negatively-charged electrons move to the surface of the N-type semiconductor, and the positively-charged electron holes move to the surface of the P-type semiconductor. Thereby, an electric potential drop appears between the P-type and N-type semiconductors, providing electric energy for the user.
Among solar cells, the monocrystalline silicon solar cell has highest photoelectric conversion efficiency and very stable performance. There have been monocrystalline silicon solar cells with a photoelectric conversion efficiency of as high as 12-22% available in the market.
Monocrystalline silicon has an energy gap of 1.12 eV. Therefore, the monocrystalline silicon solar cell can only make use of the portion of sunlight having wavelengths shorter than 1100 nm. However, there is still a large portion of sunlight having wavelengths longer than 1100 nm. Therefore, how to extract more electric energy from sunlight has been the problem that the fields concerned desire to overcome. Besides, how to use reliable and low-cost processes to obtain materials of high photoelectric conversion efficiency is another problem of the fields concerned.